The present invention relates to a color display system and particularly to a color cathode ray system including a cathode ray tube having improved resolution over the entire phosphor screen and a color display system provided with this cathode ray tube. The resolution of a color cathode ray tube depends on the size and shape of beam spots on the phosphor screen. If the beam spot formed by impingement of an electron beam emitted from an electron gun onto the phosphor screen and resultant luminescence of the phosphor screen is small in diameter and close to a true circle, it provides a good resolution.
The electron beam emitted from the electron gun is deflected horizontally and vertically on the way to the phosphor screen and reaches the phosphor screen. The central area and peripheral area of the phosphor screen are different in the distance from the center of deflecting from each other, so that as the deflection of the electron beam increases, the shape of the beam spot elongates vertically for the most part.
In a so-called in-line electron gun emitting three electron beams, the two side electron beams are displaced from the tube axis, so that their convergence is degraded in the peripheral area of the phosphor screen and the resolution deteriorates.
FIG. 1 is a schematic cross sectional view illustrating a structure example of a color cathode ray tube to which the present invention is applied. Numeral 1 indicates a panel portion, 2 a funnel portion, 3 a neck portion, 4 a phosphor screen, and 5 a shadow mask which is a color selection electrode. Numeral 6 indicates a third electrode, 7 a fourth electrode, 8 a shield cup, 14 a deflection yoke, 15, 16, and 17 center axes of electron beams, and 18 and 19 center lines of the side electron beam passage apertures of the fourth electrode 7.
Cathode portions K1, K2, and K3, a first electrode 10, and a second electrode 20 constitute a so-called triode portion.
As shown in FIG. 1, the color cathode ray tube comprises an evacuated envelope formed of the panel portion 1 and the neck portion 3 joined to the side wall of the panel portion 1 via the funnel 2, an electron gun incorporated in the neck portion 3, the deflection yoke 14 mounted on the outer wall of the funnel portion 2 and the neck portion 3 in the neighborhood of their junction, and the multi-apertured shadow mask 5 in predetermined spaced relation adjacent to the phosphor screen 4.
Striped or dotted phosphors of red, green, and blue are coated on the phosphor screen.
Three electron beams emitted from the electron gun are color-selected by the shadow mask 5, impinge on the phosphors associated with the respective electron beams and cause the phosphors to luminesce.
The electron gun comprises an electron beam generation portion for generating, accelerating, and controlling three parallel electron beams of in-line arrangement from the cathode portions K1, K2, and K3, a prefocus lens portion for focusing the electron beams slightly, and a main lens portion for focusing the electron beams on the phosphor screen 4 and the three electron beams are deflected by the magnetic deflection yoke 14 so as to scan the beams in a rectangular raster over the phosphor screen 4.
The constitution shown in FIG. 1 is an example and a variety of electron guns are known in terms of the number of electrodes constituting the electron gun, the shapes of electron beam apertures in the electrodes, and the structures of the electrodes.
FIG. 2 is an illustration of the magnetic deflection field by the deflection yoke acting on electron beams. The magnetic deflection field by the magnetic deflection yoke has, as shown in FIG. 2, a pin cushion shaped distortion 14H in the horizontal deflection field and a barrel shaped distortion 14V in the vertical deflection field.
FIGS. 3A and 3B are illustrations of the deflection and shape distortion of an electron beam spot by the magnetic deflection field. An electron beam B deflected to the periphery of the phosphor screen is subject to diffusing force fh in the horizontal direction and focusing force fv in the vertical direction as shown in FIG. 3B in addition to the force Fh for deflecting the electron beam as shown in FIG. 3A and forms a distorted spot shape.
FIG. 4 is an illustration of the beam spot shapes on the phosphor screen. Although the beam spot 00 in the center area of the phosphor screen 3 is circular, the beam spots generated in the peripheral area of the phosphor screen are distorted to a non-circle comprising a core BC of high intensity and a halo BH and particularly the large vertical elongation of the halo BH adversely affects the focus characteristic.
As a countermeasure for degradation of the focus characteristic, for example, an art disclosed in Japanese Patent Application Laid-Open 62-58549 may be cited.
FIG. 5 is a cross sectional view illustrating the constitution of the electron gun disclosed in the aforementioned prior art. Symbols K1, K2, and K3 indicate cathodes, numeral 10 a control grid, 20 an accelerating electrode, 30 a first focus electrode, 40 a second focus electrode, 48 a rim electrode, 50 a third focus electrode, 60 an anode, 11, 12, 13, 21, 22, 23, 31, 32, 33, 41a, 42a, 43a, 41b, 42b, 43b, 51a, 52a, 53a, 51b, 52b, 53b, 61, 62, and 63 respective electron beam passage apertures thereof, 44, 45, 46, and 47 vertical plates, and 54 and 55 horizontal plates. Symbol C indicates an electron gun axis (coincides with the tube axis), S1 a displacement of each of the side electron beams from the electron gun axis C, and S2 a displacement of each of the side electron beam passage apertures 61 and 63 of the anode 60 from the electron gun axis C.
FIG. 6 is a plan view of the accelerating electrode 20 in a direction of the arrow 100 shown in FIG. 5, and FIG. 7 is also a plan view of the second focus electrode 40 in a direction of the arrow 101, and FIG. 8 is also a plan view of the third focus electrode 50 in a direction of the arrow 102.
As shown in FIG. 6, slits 24, 25, and 26 elongated in the in-line direction of the three electron beams are superposed on the three circular electron beam passage apertures 21, 22 and 23 on the first focus electrode 30 side of the accelerating electrode 20.
As shown in FIG. 7, the second focus electrode 40 has the circular electron beam passage apertures 41b, 42b, and 43b on the side of the third focus electrode 50, opposes the third focus electrode 50, and furthermore has a first plate electrode (vertical plate) comprising the four vertical parallel plates 44, 45, 46, and 47 which are attached on the opposite sides of each aperture so as to extend toward the third focus electrode 50.
The second focus electrode 40 has the rim electrode 48 which surrounds the first plate electrode and extends a predetermined distance from ends 44a, 45a, 46a, and 47a of the parallel plates toward the third focus electrode 50.
As shown in FIG. 8, the third focus electrode 50 has the three circular electron beam passage apertures 51a, 52a, and 53a on the side of the second focus electrode 40 and has a second plate electrode (horizontal plate) comprising a pair of horizontal parallel plates 54 and 55 which are attached so as to sandwich the three circular electron beam passage apertures vertically and to extend toward the second focus electrode 40. The ends 54a and 55a of the horizontal parallel plates constituting the second plate electrode extend into the rim electrode 48 of the second focus electrode 40 and are spaced a predetermined interval L from the ends 44a, 45a, 46a, and 47a of the vertical parallel plates of the second focus electrode 40 along the electron gun axis.
The anode 60 has the three circular electron beam passage apertures 61, 62, and 63 on its end face. Between the displacement S2 of the side electron beam passage apertures 61 and 63 from the electron gun axis and the displacement S1 of the cathodes K1 and K3 and the side electron beam passage apertures of the control grid 10, the accelerating electrode 20, the first focus electrode 30, the second focus electrode 40, and the third focus electrode 50 preceding the anode 60, a relation of S2&gt;S1 is held, a main lens is formed between the third focus electrode 50 and the anode 60, and the side electron beams SB1 and SB2 are converged at a point on the phosphor screen.
In operation of the electron gun, 50 to 170 V is applied to the cathodes K1, K2, and K3, 0 to -150 V to the control grid 10, 400 to 800 v to the accelerating electrode 20, 5 to 8 kV to the second focus electrode 40 as a focus voltage Vf, 23 to 30 kV to the anode 60 as an anode voltage Eb, and a dynamic voltage Dvf which varies in synchronization with the horizontal and vertical deflections of the electron beams to the first focus electrode 30 and the third focus electrode 50.
When the electron beams are undeflected, there exists no potential difference between the first focus electrode 30, the second focus electrode 40, and the third focus electrode 50. Therefore, the presence of the parallel plates (vertical plates) 44, 45, 46, and 47 in the second focus electrode 40 and the parallel plates (horizontal plates) 54 and 55 attached to the third focus electrode 50 exerts no influence on the beams and the cross section of the electron beams are elongated horizontally by a quadrupole lens formed by the slits 24, 25, and 26 elongated in the in-line direction of the three electron beams on the side of the first focus electrode 30 of the accelerating electrode 20 but the electron beams are brought into an optimum focus on the phosphor screen by the main lens between the third focus electrode 50 and the anode 60.
FIG. 9 is an illustration of an electron beam bundle emitted from the accelerating electrode 20 under the aforementioned operating voltage condition and FIG. 10 is a schematic diagram expressing the electron beam trajectories electron-optically.
The electron beams leaving the slits 24, 25, and 26 of the accelerating electrode 20 are subjected to a strong vertical focusing action and the cross section of each electron beam is elongated horizontally on the phosphor screen as shown in FIG. 9. In this case, the H portion of high current density is formed in the center of each cross section and the L portions of low current density are formed on both sides thereof.
When the electron beam is undeflected, the electron trajectories are as shown in FIG. 10, and the electron beam is overfocused horizontally as indicated with Ph and underfocused vertically as indicated with Pv, due to spherical aberration and the focus voltage is adjusted for focus within the shown range W on the phosphor screen.
The beam spot on the phosphor screen at this time has a vertically elongated shape comprising the H portion of high current density.
FIG. 11 is an illustration of an effect on beam spots by the parallel plates (vertical plates) 44, 45, 46, and 47 in the second focus electrode 40 and the parallel plates (horizontal plates) 54 and 55 attached to the third focus electrode 50 and FIG. 12 is an illustration of an effect on a beam spot by the parallel plates (horizontal plates) 54 and 55 attached to the third focus electrode 50.
When the deflection amount of each electron beam is increased, the potentials of the first focus electrode 30 and the third focus electrode 50 is made higher than the potential of the second focus electrode 40. Therefore, a strong horizontally focusing lens action (Fv&lt;Fh) by the parallel plates (vertical plates) (44), 45, 46, and (47) in the second focus electrode 40 as shown in FIG. 11 and a strong vertically divergent lens action Fvv by the parallel plates (horizontal plates) 54 and 55 attached to the third focus electrode 50 as shown in FIG. 12, constitute a quadrupole lens electric field and the cross section of the electron beam is shaped to be elongated vertically, and at the same time the potential difference between the third focus electrode 50 and the anode 60 is reduced, and the focusing action by the main lens is weakened, and the electron beams are brought into an optimum focus in the peripheral area of the phosphor screen.
The aforementioned quadrupole lens action acts so as to cancel the effect on the electron beams by the magnetic deflection aberration, so that the electron beams are brought into an optimum focus on the screen However, the entrance angle of the electron beam into the main lens formed by the third focus electrode 50 and the anode 60 and the beam diameter are different between the horizontal direction and the vertical direction, and it is impossible to make the shape of the beam spot closer to a circle because the lens magnification in the main lens is different between the horizontal direction and the vertical direction.
FIGS. 13A and 13B are illustrations of a light-optical equivalent of the quadrupole lens action by the second and third focus electrodes and the electron beam trajectories when the electron beams are deflected horizontally, and FIG. 13A is a horizontal cross sectional view, and FIG. 13B is a vertical cross sectional view. Numeral 70 indicates a crossover point of an electron beam equivalent to an object of the lens system, 72 a convex lens representing the horizontal focusing action by a quadrupole lens electric field formed between the second focus electrode and the third focus electrode, 73 a main lens, 74 a concave lens representing the horizontal diverging action by the magnetic deflection field, 75 a phosphor screen, 76 an electron beam trajectory, 78 a concave lens representing the vertical diverging action, 79 a convex lens representing the vertical focusing action by the magnetic deflection field, and 80 a beam impinging point on the phosphor screen.
As shown in FIGS. 13A and 13B, the electron lens system can be represented by a light-optics equivalent of a sequential arrangement of the convex, convex, and concave lenses in a horizontal cross section from the object 70 side and a sequential arrangement of the concave, convex, and convex lenses in a vertical cross section. When the lens system is adjusted for horizontally and vertically optimum focuses, the horizontal and vertical entrance angles of the beam impinging on the phosphor screen 75 have a relation of .alpha.H&lt;.alpha.V.
Assuming that an electron beam leaves the object 70 at an exit angle .alpha. and impinges on a position 80 at the entrance angle .alpha. O on the phosphor screen via the lens system, and the potentials at the object 70 and the phosphor screen are V and V' respectively, the electron lens system magnification M can be generally expressed by M=(.alpha./.alpha.0) V/V'!.sup.1/2, and the horizontal magnification MH of the lens system can be expressed by MH =(.alpha./.alpha.H) V/V'!.sup.1/2 and the vertical magnification MV can be expressed by MV=.alpha./.alpha.V) V/V'!.sup.1/2 .
As mentioned above, the horizontal and vertical entrance angles of impinging on the phosphor screen 75 have a relationship of .alpha.H&lt;.alpha.V, resulting in the relationship of the lens magnifications MV&lt;MH, and the beam spot diameter becomes elongated horizontally.
To correct the horizontal and vertical lens magnifications, the slits 24, 25, and 26 are formed in the accelerating electrode 20 as shown in FIG. 6.
FIGS. 14A and 14B are illustrations of light-optics equivalents representing a correction of the horizontal and vertical lens magnifications by the slits of the accelerating electrode, and FIG. 14A is a horizontal cross sectional view, and FIG. 14B is a vertical cross sectional view.
As shown in FIGS. 14A and 14B, the quadrupole lens electric field generated by the slits of the accelerating electrode produces a convex lens 71 having a weak focusing action in the horizontal direction and a convex lens 77 having a strong focusing action in the vertical direction.
An electron beam emitted from the object 70 at an angle of .alpha. enters the convex lens 71 in the horizontal direction the focusing action of which is weaker than that in the vertical direction, so that the exit angle in the horizontal direction becomes .alpha.' close to .alpha. and the exit angle in the vertical direction becomes .alpha." smaller than .alpha.. In this case, the object position viewed from the electron beam having passed the convex lens 71 or 77 generally moves backward from the object 70. However, since the accelerating electrode is at the crossover position, this shift is small and can be ignored.
The exit angle of the electron beam in the vertical direction is made smaller than that in the horizontal direction by the quadrupole lens electric fields (convex lenses) 71 and 77 generated by the slits of the accelerating electrode. As a result, the vertical entrance angle .alpha.'V of an electron beam which passes through the electron lens system and strikes the beam impinging point 80 on the phosphor screen will not become excessively larger than the horizontal entrance angle .alpha.'H, and .alpha.'V can be considered to be nearly equal to .alpha.'H. Namely, the vertical and horizontal lens magnifications MV and MH can be considered nearly equal to each other.
By doing this, an optimum focus characteristic can be obtained over the entire phosphor screen.